Wafer cleaning in modern ULSI semiconductor processing presents numerous engineering dilemmas. At various stages during integrated circuit (IC) fabrication, photoresist, residues, and/or particles must be stripped and/or cleaned from wafers. In the evolution of wafer cleaning, the industry previously employed strong acids, bases, solvents, and oxidizing agents in wet chemical procedures. The wet chemical cleaning agents include such chemicals as sulfuric acid, hydrochloric acid, hydrofluoric acid, ammonium hydroxide, and hydrogen peroxide. Although these chemicals are capable of removing unwanted materials from a wafer substrate, the aqueous solutions commonly used for cleaning have difficulty moving into and subsequently leaving the small features (less than 100 nm) on advanced ICs due to surface tension limitations of the liquid. Wet clean solutions have also been shown to be difficult to remove from the pores commonly found in advanced low-k films. In addition, feature distortion, sticking, or pore collapse may occur due to surface tension effects during drying after wet processing. Another limitation of traditional wet clean processes is the large amount of highly toxic chemical waste that can be generated. New technologies are required for next generation chip manufacturing processes.
Among the methods under development are high-pressure processes that employ “densified” process solutions. Densified fluids are supercritical or near critical solvents such as supercritical carbon dioxide. Although supercritical fluids are generating interest as potential wafer cleaning agents, they present their own unique set of engineering challenges. In particular, the cleaning fluid should strongly but selectively solvate or otherwise strip and clean photoresist, residue, contaminant, or whatever else needs to be removed from the wafer.
Some researchers have suggested the addition of additives to supercritical carbon dioxide. See, e.g., U.S. Pat. Nos. 5,944,996 and 5,783,082. Other researchers have suggested the use of oxidizing agents such as oxygen. See JP Patent Document No. S64-45131. Still other researchers have proposed specific additive combinations for removing specific contaminants. See, e.g., U.S. Pat. Nos. 5,868,862, 5,868,856, 6,024,801, and 5,866,005.